Automatic balancing devices for counterbalancing out-of-balance masses in rotating bodies are known. Many work on the well-known principle that, at speeds above the critical speed of the system in which the body is rotating, freely-rotatable counterbalancing masses will automatically take up positions in which the out-of-balance mass is counterbalanced. It has also been recognised that, if these counterbalancing masses are left unconstrained at speeds below the critical speed, they exacerbate the excursion of the rotating body which is highly undesirable. In order to remove this problem, devices have been proposed in which, at speeds below critical, the counterbalancing masses are locked in a balanced position about the axis so that, instead of having a detrimental effect on the system, they have no effect at all. Examples of such systems are shown in U.S. Pat. No. 5,813,253 and GB 1,092,188.
GB 2,388,849 discloses an improved automatic balancing system suitable for use in a washing machine in which constraining means are permanently provided on the two counterbalancing masses so as to limit the separation of the masses at speeds both above and below critical. A certain amount of counterbalancing at below critical speeds can be achieved with this system. This system has merit but suffers from the disadvantage that the amount of counterbalancing achievable below the critical speed varies with time and so the point at which the speed of rotation is increased to and through the critical speed needs to be carefully controlled in order to achieve the best results. The fact that the same constraints are applied to the counterbalancing masses at speeds both above and below critical can also inhibit the effect of the masses in some cases.